Intelligent security has become a widespread and necessary reality of modern day civilization, and one aspect of known intelligent security is video surveillance. Video surveillance is an integral part of the technology used in modern day security systems.
Known security systems can include surveillance cameras or other data collection devices, video recorders or other data storage devices, and video viewers or monitors. For example, surveillance cameras can monitor a particular region, and video data streams from the cameras can be displayed and monitored by security personnel on video monitors. The video can be stored in associated video recorders.
When a monitored region is lit with artificial lighting, image flicker issues can arise. For example, many monitored regions are lit with fluorescent lamps having a frequency of approximately 50-60 Hz. When the region is captured by a surveillance video camera, flickers of frequency are often generated due to the relation between the field frequency and the lighting frequency of the lamp. Accordingly, systems and methods have been developed to reduce image flicker.
For example, many known surveillance video cameras use an n/100s shutter speed when the monitored area is lit with a 50 Hz lamp and use an n/120s shutter speed when the monitored region is lit with a 60 Hz lamp to reduce and/or eliminate the lamp's cyclical effect. Auto exposure (AE) measuring is determined by each pixel's y accumulation during each frame's exposure time.
FIG. 1A is a graph depicting lighting voltage versus time in a 50 Hz AC lighting system, and FIG. 1B is a graph depicting lighting luminance versus time in a 50 Hz AC lighting system. When a 50 Hz AC powered lamp is employed, the luminance y operates at a 100 Hz frequency. To avoid the periodic y variation effect, known systems and methods to reduce image flicker use an n/100s fixed exposure time, wherein n=1, 2, 3, 4 . . . . During the exposure time, each pixel's y accumulation will not vary between a different start point and end point. Accordingly, flicker issues can be substantially reduced and/or eliminated.
However, in known systems and methods to reduce image flicker, automatic gain control (AGC) steps are inserted between n/100s and (n+1)/100s to achieve the proper AE level. The AGC gain causes an increase in image noise and causes a decrease in image signal to noise (S/N) ratio. Further, when it is necessary to operate the shutter speed at less than 1/100s exposure time, known systems and methods have been unable to reduce bright scene flicker.
There is, thus, a continuing, ongoing need for improved systems and methods to reduce image flicker. Preferably, such improved systems and methods employ an AE statistic counter-measure.